For someone with no prior history of diabetes, an A1C level below 5.7 percent is considered normal. A1C levels from 5.7 to 6.4 percent indicate prediabetes, and levels at 6.5 percent or above indicate diabetes, according to the National Institutes of Diabetes and Digestive and Kidney Diseases.
A1C testing yields data equivalent to the average of a person's blood glucose level over the past three months, within a 0.5 percent range of error. Glucose molecules in the blood attach to the hemoglobin molecules in red blood cells. Taking a three-month snapshot from just one blood sample is possible because red blood cells have a three-month lifespan, the National Institutes of Diabetes and Digestive and Kidney Diseases explains.